Bottles made of thermoplastic material such as, for example, polyester have been widely employed as containers or bottles into which drinks or water such as, for example, fruit juice, coffee, mineral water, carbonated drinks such as, for example, cola or cider, or other similar drinks is to be contained. The formation of these plastic bottles has been performed by means of a method, that is, the so called biaxial stretching blow formation method, in which a preform made by means of an injection molding process is heated to a predetermined temperature, and the thus heated preform is extended axially within a mold by means of suitable extending rod member and simultaneously blown so as to be expanded circumferentially.
In order to heat the preform, a non-contact heating method by means of an externally disposed infrared ray heater has been generally adapted, but recently in accordance with the requirement for large-sized containers or bottles, the thickness of the preform has become large. Accordingly, in view of the increased time required for the uniform heating of the preform in the direction of the thickness of the preform, a shortening of the remaining steps of the manufacturing process thereof has been required, and thus the conventional forming method referred to above cannot satisfy these requirements. In order to obviate these defects, there is provided a method, for example disclosed in the Japanese Patent Laid-open Publication No. 261024/1986, in which an internal heater is arranged inside the preform in addition to the conventional external heater, or a method which employs a jig, for example disclosed in the Japanese Patent Publication No. 43852/1987, in which a rod is heated by means of high frequency induction heating so as to thereby heat the preform internally and externally. There is also provided a method, for example disclosed in the Japanese Patent Laid-open Publication No. 163828/1986, in which a heating pipe is inserted into the preform so as to internally heat the same.
It is required, however, with respect to bottles made of plastic materials to control the heating level during the formation process so as to achieve the most suitable thickness distribution in the axial direction thereof in order to properly maintain the strength and the shape of the bottle. For this purpose, it is required to adjust the preform heating temperature along the axial direction of the preform, but with the methods disclosed in the Japanese Patent Laid-open Publication Nos. 261024/1986 and 163828/1986 referred to above, the preform is quickly heated by means of the internal and external heating thereof, whereby it is difficult to heat the preform in accordance with the desired predetermined temperature distribution in the axial direction. Moreover, in accordance with the heating method by means of the infrared ray heater, since a plurality of heaters are arranged in the axial direction of the preform and electric power to the individual heaters is respectively regulated as occasion demands, it is difficult to perform the rapid heating of the preform and sometimes an optional portion of the preform is unintentionally heated to the predetermined temperature due to the thermal interference with the respective heaters.
In addition, with the method disclosed in the Japanese Patent Publication No. 43852/1987, a metallic rod disposed within the preform is heated by means of the induction heating, so that it is considerably difficult to measure the temperature of the metallic rod during or just before the induction heating process, whereby the heating of the metallic rod is not properly controlled. In addition, it is also sometimes difficult to suitably supply the preforms, and consequently, the temperature control of an individual metallic rod cannot be properly performed in the case where the preform is not supplied.
In order to precisely control the temperature of the preforms, it is necessary to precisely maintain the temperature of the heating member at a predetermined temperature, but it is impossible by means of the conventional methods to strictly maintain the temperature so as to remain constant because the temperature of the heating member increases as a result of the absorption of the heat energy and decreases as a result of heat radiation, or because of various factors tending to vary the heating conditions.